Self-cleaning gas oven

ABSTRACT

A GAS COOKING OVEN WHERE A MAIN STREAM OF THE HOT FLUE GASES GENERATED IN THE LOWER COMBUSTION CHAMBER IS PASSED AROUND THE OUTSIDE OF THE OVEN LINER, WHILE A SECONDARY STREAM OF FLUE GASES PASSES COMPLETELY THROUGH THE OVEN LINER AND THEN REJOINS THE SAID MAIN STREAM BEFORE THE FLUE GASES ARE PASSED THROUGH A HEAT EXCHANGER FOR EXTRACTING SOME OF THE HEAT THEREFROM. COOL ROOM AIR IS DRAWN INTO THE OVEN CABINET AND PASSED THROUGH THE HEAT EXCHANGER TO EXTRACT SOME OF THE HEAT FROM THE HOT FLUE GASES. AIR PASSAGES ARE FORMED IN HEAT TRANSFER RELATION WITH THE OUTER WALLS OF THE OVEN CABINET, AND THE HEATED AIR PASSES THROUGH THE PASSAGES AND PICKS UP MORE HEAT FROM THE WALLS OF THE CABINET BEFORE PASSING INTO THE COMBUSTION CHAMBER AS PREHEATED SECONDARY AIR FOR THE GAS BURNER MEANS. SOME COOL AIR BYPASSES THE HEAT EXCHANGER, AND PASSES DIRECTLY TO THE OVEN CABINET, WHILE OTHER COOL AIR MIXES WITH THE FLUE GASES DISCHARGING FROM THE HEAT EXCHANGER TO COOL THE GASES FURTHER BEFORE THE GASES ARE RETURNED TO THE KITCHEN ATMOSPHERE.

United States Patent [72] Inventors Wayne L. Henderson; PrimaryExaminer-Charles J Myhre Raymond L. Dills, Louisville, Ky. Anorneys-Richard L. Caslin, Harry F. Manbeck, Jr., Frank [2i] Appl. No. 859,860L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [22] Filed Sept. 22,1969 [45] Patented June 28, 1971 [73] Assign Gene, m Company ABSTRACT: Agas cooking oven where a mam stream of the hot flue gases generated inthe lower combustion chamber is passed around the outside of the ovenliner, while a secondary stream of flue gases passes completely throughthe oven liner and then rejoins the said main stream before the fluegases are [541 SELFCLEANING GAS OVEN passed through a heat exchanger forextracting some of the 14 Chins 5 Dnwhg Fm heat therefrom. Cool room arris drawn into the oven cabinet and passed through the heat exchanger toextract some of the [52] US. Cl 126/21A heat from the hot fl gases Airpassages are formed in heat F24: /32 transfer relation with the outerwalls of the oven cabinet, and

[] Field of Search 126/19, 2 l the heated air passes through thepassages and Picks up more (A), 39 273 heat from the walls of thecabinet before passing into the combustion chamber as preheatedsecondary air for the gas burner [56] References Cmd means. Some coolair bypasses the heat exchanger and passes UNHED STATES PATENTS directlyto the oven cabinet, while other cool air mixes with 3,364,912 l/l968Dills et al l26/2lA the flue gases discharging from the heat exchangerto cool the 3,417,742 12/1968 Perl t. l26/2lA gases further before thegases are returned to the kitchen at- 7 3,480,000 11/1969 Torrey et al.126/21AX mosphere.

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28 g 63 1 ,e7 -ee o l c i O i o 1; :3 I z H 65 I l g a 34 a1 PATENTEUJUN28 |97| SHEET 2 BF 3 OUTLET o 4 HEAT EXCHANGER 76 INTAKE OVEN CABINETN RO SL E V.N. EL H .D LN O mm M W w B @QMXM COMBUSTION CHAM BER THEHZATTORNEY PATENTED JUN28|97| 3587-557 SHEET 3 BF 3 INVENTORS WAYNE L.HENDERSON LL I h L. DLLS P WJKE AL;

mam ATTORNEY SELF-CLEANING GAS OVEN BACKGROUND OF THE INVENTIONteachings of the basic patent of Bodhan Hurko No. 3,121,158 whichdiscloses the controlled use of heat for automatically cleaning the foodsoils from the inner walls forming an oven cooking cavity. During thecleaning cycle an oven wall temperature somewhere between about 750 F.and about 950 F. is produced for a sufficient period of time fordegrading the food soils into gaseous degradation products. Thedegradation products are then treated by an oxidation unit or smokeeliminator for oxidizing the gases before they are returned to thekitchen atmosphere.

The Huebler et al. Pat. No. 3,416,509 relates to a design ofself-cleaning gas oven using a heat exchanger in cooperation with thegas burner means for preheating incoming air to be used for combustionin the self-cleaning oven process, while reducing the temperature of thehot flue gases before they are exhausted directly into the kitchen.

Of further background to the present invention are Pat. No. 3,364,912 ofRaymond L. Dills and Bohdan Hurko, assigned to the present assignee,which relates to a muffle oven design for applying heat externally to aself-cleaning gas oven liner, and copending Application Ser. No. 755,555of Bohdan Hurko and Raymond L. Dills, now Pat. No. 3,507,265 that isalso assigned to the present assignee, and disclosing a muffle ovendesign for a gas self-cleaning oven in combination with a heatexchanger.

The principal object of the present invention is to provide a gascooking oven with the capability of a high temperature, pyrolytic,self-cleaning oven cycle of operation wherein cooling air is used toextract some of the heat from the hot flue gases as well as from thewalls of the oven cabinet and in so doing becomes heated to advantageand in its heated state is used as preheated secondary air for the gasburner means of the oven.

A further object of the present invention is to provide a gasself-cleaning oven of the class described wherein a forced draft of coolroom air is used for cooling the outer walls ofthe oven cabinet, alsofor reducing the temperature ofthe hot flue gases by means of a heatexchanger, as well as being mixed with the flue gases exhausting fromthe heat exchanger to further lower the temperature of the gases beforethe gases are returned to the kitchen atmosphere.

SUMMARY OF THE INVENTION The present invention, in accordance with oneform thereof, relates to a self-cleaning gas cooking oven having an ovencabinet supporting both an oven liner and a front-opening access doorwhich in unison define an oven cooking cavity. A firebox surrounds theoven liner and is spaced outwardly therefrom to form external heatingchannels around the oven liner. A gas burner is located in the bottom ofthe firebox beneath the oven liner. A main stream of flue gasestransfers heat to the oven liner by convection currents and by radiationfrom the hot flue gases which flow through the heating channels and intocontact with the outside of the oven liner in a predetermined path. Aheat exchange unit is located above the firebox and there is an airmovement means for drawing cool room air into the oven cabinet andpassing it through the heat exchanger for extracting some of the heat ofthe hot flue gases that also pass through the heat exchanger but in anopposite direction. The heated air then passes into heat transferrelation with the outer walls of the oven cabinet for further raisingthe temperature of the air as well as lowering the exterior temperatureof the oven cabinet. Then this heated air is introduced into thecombustion chamber as preheated secondary air for the gas burner means.

BRIEF DESCRIPTION OF THE DRAWINGS Our invention will be betterunderstood from the following description taken in conjunction with theaccompanying drawings and its scope will be pointed out in the appendedclaims.

FIG. 1 is a right side elevational view of a gas oven for use as abuilt-in, self-cleaning wall oven with some parts broken away and othersin cross section to show the layout of the oven construction forobserving the path of the cooling air with relation to the hot fluegases, with a heat exchange unit mounted at the top ofthe oven.

FIG. 2 is a transverse cross-sectional elevational view taken on theline 2-2 of FIG. 1 looking toward the back of the oven to again show thenature of the cooling air flow and the flow of the hot flue gases.

FIG. 3 is a top plan view of the oven of FIG. 1 with the rear portion ofthe top wall of the oven cabinet removed to show the fan for drawingcool room air into the oven cabinet and dividing it into three streams;a main stream that passes through the heat exchanger, a secondary streamthat passes directly into heat transfer relation with the outer walls ofthe oven cabinet, and a secondary stream that is diverted from the heatexchanger and mixed with the flue gases being exhausted from the heatexchanger before the gases are returned to the kitchen atmosphere.

FIG. 4 is a cross-sectional top plan view similar to that of FIG. 3 buttaken on the line 4-4 of FIG. I at a lower elevation, with part of theheat exchanger broken away to uncover the smoke eliminator I10therebeneath.

FIG. 5 is a diagrammatic flow chart of the oven cabinet cooling and ovenheating systems of a gas self-cleaning oven embodying the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to a considerationof the drawings and in particular to FIG. I, there is shown a built-in,domestic gas oven 10 in right side elevational view comprising a sheetmetal outer cabinet or body structure 11 of boxlike configuration havinga bottom wall 12, a back wall 13, a top wall 14 and finally oppositesidewalls 15,15 as are seen in the front view of FIG. 2. There is anoven cooking cavity 17 formed by a boxlike oven liner l8 and afront-opening access door 20. The door has a handle 21 for ease inmoving the door. The oven liner has a bottom wall 22, a back wall 23, atop wall 24 and opposite sidewalls 26,26. The bottom wall 22 of the ovenliner is formed with a large rectangular opening 28, that is adapted tobe substantially closed by a removable cover plate 29 which overlies thesame.

Located within the oven cabinet 11, and surrounding the oven liner 18,is a sheet metal firebox 31 which is spaced outwardly from the ovenliner to form narrow heating channels around the oven liner for thepassage of convection currents of hot flue gases therethrough in themanner of a muffle oven. This firebox 31 is deepened at the bottom toform a combustion chamber 33 for accommodating one or more gas burnertubes 34. In addition to the walls forming the combustion chamber 33,the firebox 31 has opposite sidewalls 35,35 and a top wall 36 andbackwall 37 as seen in FIG. 1. Thus, the firebox 3] creates two sideheating channels 40,40, a rear heating channel 41 and a top heatingchannel 42.

The reason for the removable cover plate 29 in the bottom of the ovenliner is to provide easy access from within the oven liner to the gasburner tube 34 within the combustion chamber 33. It is also necessary toshield the cover plate 29 from the burner 34 so that the plate does notbecome over heated due to its close proximity to the burner. We havechosen to use a second removable shield 45 which is seated beneath theopening 28 and is supported at its corners on brackets or ledges 47, asshown in FIG. 2. This heat shield 45 also should be removable, and itmay be lifted out through the opening 28 in the bottom wall 22 of theoven liner by first shifting it sideways partially off the ledges 47 andthen picking it up through the opening.

The front of the oven liner 1!! is open, and it has an outwardly turnedflange 49 on its front peripheral edge formed as part of the supportingmeans for the oven liner. Tension means (not shown) such as adjustableJ-bolts or the like are used at the rear of the oven to pull the ovenliner 18 back into the oven cabinet 11 until the flange 49 bears againsta surrounding collar 50 that represents the front frame of the ovencabinet 11. Sandwiched between the flange 49 and the front frame 50 is athermal breaker 52 such as a continuous asbestos or fiber glass gasketwhich serves to break the path of the heat flow by conduction from theoven liner 18 to the front of the oven cabinet 11, as well as to providea sealing means between these two elements and prevent flue gases fromescaping from the front of the oven.

The oven door is of a design for specific use with a high temperatureself-cleaning oven. By that is meant that the door is heavily insulatedso as to retain most of the heat within the oven cavity 17 so that thetemperature of the outer surface of the door will be within acomfortable temperature range so as not to cause injury to those in thehousehold who might come in contact with the door, as well as to preventthe wasteful loss of heat into the kitchen. The door 20 is shown with anouter door panel 55 ofshallow pan shape and an inner door panel 56 whichis adapted to be carried thereby but thermally isolated therefrom bymeans of a high temperature gasket 58 such as woven asbestos or the likethat is sandwiched therebetween. This gasket serves to bear against thefront frame 50 of the oven cabinet 11 to substantially close the frontopening ofthe oven. For more detailed information on a suitable designof a door for the oven of the present invention, reference may be madeto the patent of Clarence Getman No. 3,189,020 which is entitled OvenDoor With Floating lnner panel," and is likewiseassigned to the assigneeofthe present invention.

The various manual control components of the oven are located in acontrol panel 60 positioned at the top of the oven cabinet directly overthe oven door 20. Such components are generally identified as element61, it being understood that such components would include oven burnercontrol valves, an oven thermostat, an oven timer control and possiblyan automatic meat thermometer unit; all as are well known in this art.

There is an oven door latching mechanism 63, shown diagrammatically asonly a door latch handle, for locking the oven door 20 in the closedposition and preventing the unlocking thereof while the oven temperatureis above a predetermined maximum cooking temperature of about 600 F.Such a door latching mechanism does not form part of the presentinvention, therefore, it is neither shown nor described in detail. Onesatisfactory design is taught in the patent of Clarence Getman No,3,l89,375 which is likewise assigned to the present assignee.

The gas burner tube 34 is shown of a size and rating somewhere betweenl8,000 and 30,000 BTUs per hour for use with a standard gas dischargenozzle and gas pressure. There may be one burner or two burners as amatter ofchoice. A single burner would be conventional for a standardgas cooking oven. However, more heating energy is necessary in a gasself-cleaning oven than in a standard gas cooking oven. The burner 34 issupported at its front end on an angle bracket 65. The opposite end ofthe burner tube 34 has a venturi tube throat 67 which extends outthrough an opening 68 toward the back of the combustion chamber 33 andthen through a duct 69 which is open to the kitchen atmosphere at therear of the oven, as is best seen in FIG. 1, where a gas connectionwould be made. Primary air for the burner is obtained from the kitchenatmosphere through this duct 69 as relatively cool air at room ambienttemperatures.

The necessary pilot and gas control valves are not shown -'nor are thetemperature sensing and control systems since they may be ofconventional design and do not form part of the present invention. Abroil burner is not illustrated in the drawings, but it could be addedin the top of the oven cavity 17 for use during normal broilingoperations, however this is merely a matter of preference.

The lower gas burner in a standard gas baking oven normally has a ratingof about 20,000 BTU's per hour which is equivalent in electrical termsto 5,850 watts of electrical power. In order to reach the properheat-cleaning temperature, which is preferably somewhere between about750 F. and about 950 F. it would be necessary to increase the size ofthe burner to about 28,000 BTUs per hour or to an electrical equivalentof about 8,200 watts. During a standard baking operation the burneroperates only for a short time at full rating, as for example ID to 15minutes during entire baking operation which may extend as much asseveral hours. However, during the heat-cleaning cycle the preheatperiod would be approximately 50 to 60 minutes with the burner operatingat full rating, and the oven is then maintained at clean temperaturesfor up to 3 hours or longer depending upon the amount of food soillodged on the walls of the oven liner and inner door. The rate ofheat tomaintain the standard baking oven at the maximum heat-cleaningtemperature for a period of from I to 3 hours would be about 75 percentofthe preheat rate or about 2 l ,000 BTU's per hour, which when comparedwith electric heating is about 6,150 watts.

According to the above analysis, if we compare heat output between astandard gas oven and an electric oven during the heat cleaning cycle,the gas oven will produce approximately four times as much heat in thekitchen as an electric oven will, on the assumption that the heatabsorbed in the oven system during cleaning is the same for both gas andelectric ovens; namely, about 5,800 BTUs per hour. The reason for thisis due to the method of heat transfer in a gas oven. Since most ovenbake gas burners arelocated in a firebox outside of the oven cavity, theheat transfer to the oven takes place mostly by convection air currentsheated by the burner. Heat transfer by convection currents of hot fluegases results in a low Bffi-.

ciency of 10 to 15 percent of heat transfer to the oven liner. in astandard gas oven large amounts of flue gases and hot air pass throughthe oven. The heating value of one pound of natural gas is about 20,000to 23,000 BTU's per hour, depending upon the particular gas used, Forideal combustion it is necessary to have at least 15,3 pounds of air foreach pound of gas. However, an excess amount of air is needed in heatingup the oven; up to 300 percent of excess air, which means for each poundof gas up to about 61.2 pounds of air 15.3 times 4) may be needed toprovide complete combustion, while holding the amount ofcarbon monoxideto a minimum.

If the flue gases are at the necessary temperature of about 900 F. abovethe room ambient temperature (At) when the gases break contact with thewalls of the oven liner during the cleaning cycle, and the heating rateis l pound of gas burned per hour, then the heat of exhaust may becalculated as follows:

per hour (approximately) m,,=pounds ofgas m,,=pounds of airAt=temperature differential The exhaust heat of 14,650 BTUs per hour isequivalent to heating a kitchen with an electric heater of 4,300 watts.In addition to this amount of heat there will be the usual heat lossesfrom the oven cabinet which will be of the same order of magnitude asthe heat losses in an electric oven and will depend upon the maximumsurface temperature of the oven cabinet.

ln an electric oven, the amount ofcirculated air through the oven isvery low, on the order of less than 3 cubic feet per minute, as comparedwith a gas oven with about 38.6 cubic feet per minute. Hence, the heatexhausted by an electric oven would be less than 8 percent of the heatexhausted by a gas oven during the heat-cleaning cycle. It has beencalculated that the efflciency of the heating system of the early gasselfcleaning ovens would be very low, on the order of less than 30percent.

Our present invention, which introduces an improved air system,including a heat exchanger, to a gas self-cleaning oven, has the effectof substantially increasing the efficiency of the heating system of theoven. Theoretically, the efficiency of the gas heating system could beincreased from about 28.3 percent up to about 85 percent when using theoven with heat exchanger and air flow system of the present invention.This has the advantage of a saving in fuel, but the saving in the costcabinet and becoming heated thereby. It thus becomes preheated secondaryair for the gas burner 34.

A second secondary stream of cool air bypasses the heat exchanger 70 bypassing directly into the top horizontal air 96, 83, 86, 90 and finallyenters into the combustion chamber 33 after removing some of the heat inthe walls of the oven of fuel will not be the only factor. With theheating system 5 channel 96 and being deflected forward through anopening using much less fuel, the amount of heat in the exhaust will be103 by means of a diverter plate 97. This cool air divertedproportionally smaller as is shown in the Table below. through theopening 103 mixes with the hot flue gases coming Heatabsorbed in systemTotal heat Heat in Eflicieuey, duringcleaning, input, Burned fuel,Burned air, Exhaust, exhaust percent B.t.u./hr. B.t.u.lhr. LbJhr. lbJhr.lb./hr. B.t.u./ht'

It will be understood that a gas oven using our invention will upthrough the exhaust opening 98 from the heat exchanger probably notreach the maximum efficiency of 85 percent 70. Thus the hot flue gasesare mixed with and cooled by the shown in this theoretical exampleabove, but the efficiency secondary stream of cool air before they arereturned to the will be substantially increased over the approximately30 perkitchen atmosphere through an air outlet opening 104. centefficiency of the standard oven. The gas burner 34 generates hot fluegases having a main Our improved heat exchanger and air flow systemwhereby stream that passes through the firebox 31 around the outside theburner efficiency is increased and the heat losses reduced of the ovenliner 18 by way of the two side heating channels will now be explained.As seen in the drawings, the system in- 40,40 and the rear heatingchannel 41 into the top heating corporates a heat exchanger 70 which isshown assembled channel 42. There is an opening 95 in the top wall 36 ofthe above the firebox 31 directly behind the control panel 60. firebox31 so that the hot flue gases pass through the opening Such a heatexchanger is for recovering much of the heat from 95 and then dispersethrough the heat exchanger 70 by passing the hot flue gases before thegases are exhausted to the between the bank of tubes 72 between the twoend plates 73 kitchen atmosphere, and at the same time preheating astream and 74. The top of the heat exchanger 70 is closed with a of coolroom air when it passes through the heat exchanger cover plate 94 exceptfor an opening 98 in the front thereof. and before it is supplied to thecombustion chamber 33 as pre- The diverter 97 is formed above thisopening 98 for catching heated secondary air for the gas burner 34. Theheat some of the cool air from the fan 77 passing through the topexchanger 70 comprises a series or bank of tubes 72 which are horizontalair channel 96 and mixing it with the flue gases in mounted at each endthrough apertured plates 73 and 74 this area. There is a duct 102 whichconnects with an air outlet respectively. Relatively cool room air isdesigned to pass opening 104 so that the flue gases may be returned tothe through the hollow tubes 72 of the heat exchanger 70. There is 3 5kitchen atmosphere at a reduced temperature. an air inlet opening 76into the oven cabinet 11 in the form of Most of the hot flue gases fromthe combustion chamber 33 an apertured panel or grill work arrangedalong the bottompass around the outside of the oven liner 18, but asecondary edge of the control panel 60. Between the heat exchanger 70stream of flue gases is allowed to flow through the cooking and the airinlet opening 76 is positioned a fan 77 comprising a cavity 17. Theremovable cover plate 29 in the bottom of the small electric motor 78and fan blades 79. When this fan 77 is oven liner 18 is provided with aseries of elongated slots 106 energized it is designed to draw cool roomair into the oven along the front edge of the plate. This passage offlue gases cabinet through the air inlet opening 76. Downstream of theinto the oven cavity provides a supplement to the heatlost fan 77 is aplenum chamber 80. The main stream of cool air through and around theoven door 20 so as to insure that the from the plenum chamber isarranged to pass through the holtemperature of the inner surface of thedoor has substantially low tubes 72 of the heat exchanger 70, while asecondary the same operating temperature as the walls of the oven linerstream is designed to pass in heat transfer relation with the so as toobtain a generally uniform wall cleaning capability. outer walls formingthe oven cabinet 11, as is best seen in FIG. The principle problem areain many self-cleaning ovens has 2. been a failure to clean properly thelower edge of both the Notice in FIG. 2 that a layer of thermalinsulation 81 is ardoor and the adjacent portion of the oven liner. Thisproblem ranged against the outside of the two sidewalls-35,35 of thedoes not exist in the design of the present invention where adfirebox31. Thus at each side of the oven between the insuladitional heat isbiased towards the front ofthe oven. tion 81 and the sidewall 15 of theoven cabinet there is formed An oven vent opening 108 is formed in thetop wall 24"of a vertical air channel 83. Looking at FlG. 1,there isshown a the oven liner 18 so that the flue gases and the gaseous layerof insulation 85 arranged against the backwall 13 of the degradationproducts created from the food soil being oven cabinet and spaced fromthe backwall 37 of the firebox removed from the oven liner may beexhausted therefrom. 31 to form a vertical air channel 86. Notice thelower end of Positioned over the vent opening 108 is a catalyticoxidation the rear air channel 86 is provided with a vent opening 88unit 110 which may comprise a hollow housing 111, as is best which joinswith the combustion chamber 33. Moreover, the seen in FIG. 2, with acentral opening 115 in its bottom wall, a lower ends of the two side airchannels 83,83 empty into a botcorrugated ceramic block 112 on eitherside of the opening tom air channel 90 which underlies the combustionchamber and a cover plate 113 seated on top of the blocks. There is an33 and is joined therewith through an elongated opening 91 in outletopening 114 in the top of the housing 111 above the the bottom wall ofthe firebox located directly beneath the gas cover plate 113 such thatthe gas flow through the smoke burner tube 34. A layer of thennalinsulation 93 is placed over eliminator 110 is up through the bottom andthen horizontally the bottom wall 12 of the oven cabinet. in oppositedirections through the pair of corrugated ceramic As to the top portionof the oven cabinet 11, notice in FIG. blocks 112,112 which have a highratio of surface area to 2 there is a cover plate 94 which overlies theheat exchanger volume and are coated with a thin layer of a catalyticmaterial 70. This cover plate serves to form a top horizontal airchannel to serve as a self-sustaining gas burner when it is heated to a96 which is open at the right side by means of slots 8 to the hightemperature by the flue gases passing therethrough. A plenum chamberformed behind the fan 77. The channel is satisfactory design of suchsmoke eliminator is taught in the open at its other end to the left sideair channel 83 by means of 70 copending application of Wayne L.Henderson, one of the slots 82, as well as to the rear channel 86, as isseen at the top present applicants, in Ser. No. 707,056, which islikewise asrear corner of FIG. 1. Hence to summarize, some of the coolsigned to the present assignee. This smoke eliminator 110 is air drawninto the oven cabinet by the fan 77 bypasses the heat positioned beneathan opening of the top wall 36 of'the exchanger 70 and moves in heattransfer relation with the firebox 31 as can be seen in FIGS. 2 and 4.Thus the main outer walls of the oven cabinet 11 through the airchannels 83, 75 stream of flue gases rising up the heating channels 40,40 and 41 surrounding the oven liner mixes with the exhaust from thesmoke eliminator before passing through the heat exchanger 70.

For a more comprehensive understanding of both the oven cabinet coolingand the oven heating system of this oven construction, attention isdirected to the diagrammatic flow chart of FIG. where reference numeralsare applied to some of the elements of the chart to designate elementsofthe oven which cooperate in affecting the proper flow patterns. Forexample, the elements are an air intake opening 76, the fan 77, the heatexchanger 70, the oven cabinet 11, the combustion chamber 33 and theoven liner 18 with its smoke eliminator 110 and finally the outletopening 104. There are times when either the air or flue gas flows aredivided into a major flow and a minor flow hereinafter referred to as amain stream and a secondary stream. A main stream is depicted by a wideline, while a secondary stream is shown as a thin or single thicknessline. For example, the cool room air drawn through the intake 76 by thefan 77 is divided into three streams; a main stream 120 and twosecondary streams 122 and 124. The main stream 120 passes through thetubes 72 of the heat exchanger 70. The first secondary stream 122bypasses the heat exchanger 70 and moves in heat transfer relation withthe outer walls of the oven cabinet 11 through the two side coolingchannels 83,83 the top cooling channel 96, the rear channel 86 and thenthe bottom channel 90. Simultaneously, the main stream of now heated air128 exhausts from the heat exchanger 70 and also passes into heattransfer relation with most of the walls of the oven cabinet such as theleft side cooling channel 83, the rear channel 86, and the bottomchannel 90. Then the heated air passes as a main stream 126 into thecombustion chamber 33 through the slot 91 in the bottom wall of thecombustion chamber and through the opening 88 in the lower portion ofthe rear channel 86. Hot flue gases are generated in the combustionchamber and pass as a dual main stream 130,130 around the oven linerthrough the heating channels 40, 40 and 41, while a secondary stream 132passes into the oven cavity through the slot 106 adjacent the front ofthe oven, and out through the smoke eliminator 110 for mixing with themain streams 130,130 before passing through the heat exchanger 70 aroundthe bank of tubes 72 and out of the heat exchanger as main stream 134.Notice that the second secondary stream 124 of cool room air alsobypasses the heat exchanger 70 for mixing with the main stream 134 ofhot flue gases coming from the heat exchanger 70 and finally passing outof the outlet opening 104.

As a modification of the present invention, instead of a muffle ovendesign for both cleaning and cooking, it is possible to convert the ovento a flow-through oven for the cooking operations by providing slots 140and an enlarged damper 142 along the lower edge of each side 26,26 ofthe oven liner 18. Each damper 142 is pivotally mounted along its upperedge and it may be shifted by a linkage mechanism (not shown) from aclosed, heat-cleaning position of FIG. 2 to an open, cooking positionwhere the dampers would close or seal the bottom of the heating channels40,40 so that the hot flue gases would be precluded from passing aroundthe oven liner 18 but would be forced through the cooking cavity 17 andout the oven vent opening 108. A similar damper (not shown) would belocated at the bottom of heating channel 41 for closing this channelduring a baking or broiling operation within the oven. No slots throughthe oven liner cooperating with the rear damper would be necessary in aflow-through gas cooking oven.

Having described above our invention of a gas self-cleaning oven with animproved air flow system and heat exchanger and a method of operatingsame, it should be readily apparent to those skilled in this art that wehave been able to increase the efficiency of combustion of the heatingsystem and hence reduce the consumption of fuel, for the same amount ofheating value in the oven liner. in particular, we have been able torecover a large amount of the heat energy from the flue gases that wouldotherwise be lost through the oven exhaust. Accordingly, the temperatureof the flue gases will be reduced to a comfortable level. In addition,the room air drawn into the oven cabinet is directed into contact withthe outer walls of the cabinet for deriving heat from the cabinet andrestricting the maximum exterior temperatures of the cabinet. This tooimproves the efficiency of the system.

lt should be apparent to those skilled in this art that we havedescribed what, at present, is considered to be the preferredembodiments of this invention in accordance with the Patg t StatutesChanges may be made in the disclosed oven without departing from thetrue spirit and scope of this invention.

We claim:

1. The method of cleaning food soils from the interior surfaces of bothan inner oven liner and an access door defining an oven cooking cavitywithin an oven cabinet where the food soils are accumulated thereuponduring the previous carrying out in said oven cavity of food cookingoperations in the normal food cooking temperature range extending fromabout F. to about 550 F., wherein said oven liner is surrounded by anouter oven liner defining a firebox spaced from the oven liner, and aheat exchanger located at the top of the oven cabinet; said methodcomprising drawing room air into the oven cabinet, passing the mainstream of room air through the heat exchanger for taking heat from theexchanger, directing the heated room air into contact with the outerwalls of the oven cabinet for deriving heat from the cabinet andrestricting the exterior temperature of the cabinet, and then supplyingthis heated room air to a gas burner in a combustion chamber beneath theoven ling as preheated secondary air to form hot flue gases, a mainstream of the flue gases from the burner being directed up around theoutside of the inner oven liner between the inner oven liner and thefirebox, while a secondary stream of the flue gases is allowed into theoven cavity for passage therethrough, discharging the secondary streamof flue gases from the oven liner and combining both streams of fluegases before they are directed through the heat exchanger to transfersome of its heat to the said cool room air, so as to reduce thetemperature of the flue gases before they are' returned to the kitchenatmosphere.

2. The method of cleaning food soils from the interior surfaces of anoven liner defining an oven cooking cavity within an oven cabinet wherethe food soils are accumulated on such surfaces during the previouscarrying out in said oven cavity of food cooking operations in thenormal food cooking range extending from about l50 F. to about 550 F.,wherein said oven liner is surrounded by a firebox having a combustionchamber beneath the oven liner, and a heat exchanger positioned over thefirebox; said method comprising drawing cool air into the oven cabinet,passing a main stream of cool air through the heat exchanger forderiving heat therefrom, directing the heated air exhausting from theheat exchanger over the inner surface of the oven cabinet both forraising the temperature of the heated air and lowering the temperatureof the outer surface of the oven cabinet, and then supplying this heatedair to a gas burner in the combustion chamber as preheated secondary airto create hot flue gases, simultaneously directing the flue gases fromthe burner around the outside of the oven liner and through the fireboxand thence through the heat exchanger to transfer some of the heat ofthe flue gases to the said cool air passing through the heat exchanger,and mixing the flue gases with a stream of cool air to further reducethe temperature of the flue gases before exhausting the gases to thekitchen atmosphere.

3. The method as recited in claim 2 wherein a small portion of the fluegases from the burner is passed into and throughout the oven liner andthen discharged from the oven liner and mixed with the flue gases beingdirected around the outside of the oven liner.

4. In a gas heated built-in wall oven, the method of cleaning food soilsfrom the interior surfaces of both an oven liner and a front-openingaccess door defining an oven cooking cavity where the food soils areaccumulated thereupon during the previous carrying out in said ovencavity of food cooking operations, wherein said oven liner is surroundedby an insulated firebox having a combustion chamber located beneath theoven liner and a heat exchanger located above the firebox, there beingan exterior oven cabinet surrounding the assembly; said methodcomprising the step of operating a forced.

draft system for drawing cool room air into the oven cabinet in the formof a main stream and at least one secondary stream, the step of passingthe main stream of cool air through the heat exchanger to be heatedthereby, then passing the heated air into contact with the outer wallsof the oven cabinet for both reducing the exterior temperature of thecabinet and raising the temperature of the heated air, and thensupplying this heated air to a gas burner in the combustion chamber aspreheated secondary air to generate hot flue gases that are divided intoa main stream and a secondary stream, the main stream of flue gasesbeing confined by the firebox and directed over the outside of the ovenliner, while the secondary stream of flue gases pass throughout the ovencavity for heating the walls forming the oven cavity into the heatcleaning temperature above about 750 F., and discharging the flue gasesfrom the oven cavity and from the firebox, passing the flue gases intothe heat exchanger for transferring heat to the said cool air enteringthe heat exchanger, and then exhausting the flue gases from the heatexchanger and mixing the flue gases with the said secondary stream ofcool air before the flue gases are returned to the kitchen atmosphere ata reduced temperature.

5. A gas cooking apparatus comprising an outer oven cabinet with asubstantially boxlike oven liner and a frontopening access door havingwalls defining an oven cooking cavity, an insulated firebox spacedaround the oven liner, gas burner means positioned in a combustionchamber in the bottom of the firebox beneath the oven liner, the gasburner means serving to generate flue gases which circulate through thefirebox for heating the walls forming the oven cavity, a heat exchangerunit means for drawing cool room air into the oven cabinet and passingit through the heat exchanger for raising the temperature of the coolair, means for passing the heated air leaving the heat exchanger intocontact with the oven cabinet for further raising the temperature of theair as well as lowering the exterior temperature of the oven cabinet,means for introducing the heated air to the combustion chamber aspreheated secondary air for the gas burner means, means for passing theflue gases from the gas burner means through the heat exchanger towithdraw some of the heat of the flue gases and transfer it to theincoming cool air, and then means for exhausting the flue gases to thekitchen atmosphere.

6. A gas cooking apparatus as recited in claim with a diverter arrangedto siphon off some of the cool air before it reaches the heat exchangerand to mix it with the hot flue gases exhausting from the heat exchangerto reduce the temperature of the gases before they reach the kitchenatmosphere.

7. A self-cleaning gas oven comprising walls forming an outer ovencabinet, a boxlike oven liner positioned in the cabinet, a front-openingaccess door for closing the oven liner and forming an oven cookingcavity, a firebox surrounding the oven liner and spaced outwardlytherefrom to form a combustion chamber beneath the oven liner with gasburner means located therein, vertical heating channels and a topheating channel communicating with the combustion chamber, a layer ofinsulation on the bottom of the oven cabinet, a layer of insulationcovering each of the two opposite sidewalls of the firebox, a layer ofinsulation on the backwall of the oven cabinet, cooling air channelsformed across the insulated bottom wall, inside each of the oppositesidewalls of the oven cabinet, inside the top wall of the oven cabinetand outside the backwall of the firebox, a heat exchanger arranged abovethe top of the firebox, an air inlet opening adjacent the top of theoven cabinet, a blower adjacent the air inlet for passing cool room airthrough both the heat exchanger and through the air channels so that theair becomes heated thereby, the heated air being exhausted into thecombustion chamber as preheated secondary air, the said gas burner meansgenerating hot flue gases which pass around the oven liner through theheating channels formed between the oven liner and the firebox, the fluegases passing from the firebox and through the heat exchanger, and anoutlet opening for discharging the gases to the kitchen atmosphere.

A slfzsls n aiqtsn rssit q. 1t im l a diverter downstream of the blowerfor bleeding off some of the cool room air and mixing it with the fluegases exhausting from the heat exchanger for lowering the temperature ofthe flue gases before they return to the kitchen atrnos here.

9. A self-cleaning gas oven as recited in claim wherein an opening isformed in the bottom of the oven liner whereby some of the hot fluegases pass into the oven cooking cavity, an oven vent adjacent the topof the oven liner for exhausting the flue gases therefrom and into theheat exchanger.

10. A self-cleaning gas oven having walls forming an outer oven cabinet,an oven cooking cavity formed by a boxlike oven liner and afront-opening access door, a firebox surrounding the oven liner andspaced outwardly therefrom, the lower portion of the firebox beingenlarged to form a combustion chamber enclosing a gas burner meanstherein, means for supplying gas and ambient primary air to the gasburner means, hot flue gases formed by the gas burner means, a mainstream of flue gases passing around the oven liner, a secondary streamof flue gases passing into the oven liner at the bottom thereof and outthe oven liner at the top to mix with the main stream of flue gases, aheat exchanger for receiving the flue gases therethrough and taking heattherefrom so as to reduce the temperature of the flue gases, an airoutlet opening in the oven cabinet for discharging the flue gases to theatmosphere, a room air inlet opening in the oven cabinet, blower meanswithin the cabinet for drawing room air into the cabinet and through theheat exchanger for receiving some of the heat from the hot flue gases,air passages in heat transfer relation with the oven cabinet, the heatedroom air from the heat exchanger passing through the said air passagesthereby reducing the outer surface temperature of the oven cabinet andthen discharging the heated air into the combustion chamber as preheatedsecondary air.

11. A self-cleaning gas oven as recited in claim 10 wherein a secondarystream of cool room air bypasses the heat exchanger and passes directlyto the air passages throughout the oven cabinet.

12. A self-cleaning gas oven as recited in claim 10 wherein a secondarystream of cool room air bypasses the heat exchanger and mixes with thehot flue gases exiting from the heat exchanger and further reduces thetemperature of the flue gases before they pass through the said airoutlet opening in the oven cabinet.

13. A self-cleaning gas oven as recited in claim 10 wherein a secondarystream of cool room air bypasses the heat exchanger and passes directlyto the air passages throughout the oven cabinet, while a secondsecondary stream of cool room air also bypasses the heat exchanger andmixes with the hot flue gases passing from the heat exchanger andfurther reduces the temperature of the flue gases before they passthrough the said air outlet opening in the oven cabinet.

14. A gas cooking apparatus comprising an outer oven cabinet with anoven liner defining an oven cooking cavity, an insulated firebox spacedaround the oven liner, gas burner means positioned in the bottom of thefirebox beneath the oven liner, the gas burner means serving to generateflue gases for heating the walls forming the oven cavity, a heatexchanger unit located above the firebox for receiving flue gasestherefrom, blower means for bringing cool room air into the oven cabinetand passing it through the heat exchanger for cooling the flue gases,means for exhausting the cooled flue gases from the heat exchanger tothe atmosphere, and means for passing the air leaving said heatexchanger into heat transfer relation with the oven cabinet for loweringthe temperature of the exterior of the oven cabinet, and means for thenintroducing this heated air over the gas burner means as preheatedsecondary air, whereby the overall efficiency of the oven is increasedconcomitantly with reductions in the exhausting flue gas temperature andthe outer cabinet temperatures.

